The Fourier analysis which is well known as FFT (fast Fourier Transformation) is one form of harmonic analysis in which frequencies to be analyzed have a harmonic relationship represented by n/T (n=1, 2, . . . ) where T is the interval of a waveform. Periodgram is a known method for period analysis, however, this requires a waveform of sufficiently longer interval than the period for analysis and cannot attain accuracy unless the periodicity of the waveform is relatively prominent. This is also true of the correlation method. The auto regression model or auto regression moving average (ARMA) model that is used for time series analysis corresponds to a digital lowpass or bandpass filter and requires a waveform of sufficient interval as in the above approaches. Prony's method, which is a form of non-harmonic analysis, cannot make an accurate analysis when noise is present.